The heart of a computer is a magnetic hard disk drive (HDD) which typically includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and/or write heads over selected data tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The volume of information processing in the information age is increasing rapidly. In particular, it is desired that HDDs be able to store more information in their limited area and volume. A technical approach to meet this desire is to increase the capacity by increasing the recording density of the HDD. To achieve higher recording density, further miniaturization of recording bits is effective, which in turn typically requires the design of smaller and smaller components.
The further miniaturization of the various components, however, presents its own set of challenges and obstacles. The ongoing quest for higher storage bit densities in magnetic media used in disk drives has reduced the size (volume) of data cells to the point where the cell dimensions are limited by the grain size of the magnetic material. Although grain size can be reduced further, there is concern that data stored within the cells is no longer thermally stable, as random thermal fluctuations at ambient temperatures are sufficient to erase data. This state is described as the superparamagnetic limit, which determines the maximum theoretical storage density for a given magnetic media. This limit may be raised by increasing the coercivity of the magnetic media or lowering the temperature. Lowering the temperature is not a practical option when designing hard disk drives for commercial and consumer use. Raising the coercivity is a practical solution, but requires write heads employing higher magnetic moment materials which will make data recording more challenging.
One additional solution has been proposed, which employs heat to lower the effective coercivity of a localized region on the magnetic media surface and writes data within this heated region. The data state becomes “fixed” upon cooling the media to ambient temperatures. This technique is broadly referred to interchangeably as “heat assisted magnetic recording” (HAMR) or “thermally assisted (magnetic) recording”, TAR or TAMR. HAMR can be applied to both longitudinal and perpendicular recording systems, although the highest density state of the art storage systems are more likely to be perpendicular recording systems. Heating of the media surface has been accomplished by a number of techniques such as focused laser beams or near field optical sources.
Some implementations of HAMR employ a near field transducer (NFT) which is used to focus optical light from an optical light source down to a spot size on the order of tens of nanometers. An optical waveguide is used to channel the optical light from the optical light source to the NFT. The focused optical light is applied to magnetic media such that the spot size heats a localized region of the magnetic medium, thereby lowering the effective coercivity thereof. U.S. Pat. No. 6,999,384 to Stancil et al., which is herein incorporated by reference, discloses near field heating of a magnetic medium.
However, the optical light source receives optical feedback from the waveguide and NFT in the form of reflected optical light. It has been well established that output power of optical light sources will become unstable in the presence of even a small optical feedback. More specifically, stochastic mode hopping events in multimode laser diodes will result in random fluctuations in the output power. This is particularly undesirable, as maintaining a constant output power is important to create uniform bit sizes for the HAMR based magnetic recording.
Although reflection produced at the interface between the optical light source and the waveguide input has been suppressed by employing anti-reflection coating of dielectric layers, suppressing reflection arising from the NFT has not been achieved in previous attempts. Accordingly, a HAMR head having near zero reflection without relying on sophisticated structural optimization of the NFT and/or without compromising other performance characteristics is desired.